When ultrasonically welding, the materials to be welded together are worked mechanically by moving the end of an ultrasonic horn up and down in the material with a frequency that lies within the ultrasonic range. Heat is generated in the material as a result of the internal friction created by this mechanical working process, causing the material to melt in the worked area so that materials located in between horn and anvil will melt and therewith fuse together. Naturally, the heat generated in the materials will depend on the degree to which the material is worked, and if the material is worked to an excessively low degree the bond between the materials will be weak or non-existent, whereas if the material is worked to an excessive degree the materials will be perforated. The anvils are often comprised of projections which stand-out from the surface of an anvil holder, so as to provide a small "contact surface" between horn and anvil, and the energy delivered by the welding unit is concentrated over a small area. When welding moving webs of material, the anvils often comprise a suitable pattern of projections on a pattern cylinder or drum. One problem encountered when ultrasonic welding moving webs, for instance when bonding together the outer sheets of absorbent articles, such as diapers, sanitary napkins or incontinence guards, is that of providing a weld seam at the high rates of web travel that are desirable in such manufacture.
U.S. Pat. No. 4,414,045 teaches an arrangement for producing nonwoven fabric by ultrasonic welding of the fibers in a mat of loosely-laid weldable fibers, this mat being moved at speeds greater than 50 m/min. This arrangement includes a plurality of anvil rolls which are disposed sequentially in the movement direction of the mat and each of which has an anvil density such that the sum of the anvils per unit area on all cylinders will correspond to the desired number of bonding points per unit area of the nonwoven fabric. An arrangement of this kind is bulky and requires the anvil cylinders to be driven synchronously in order to achieve a desired bonding pattern.